1. Field of the Invention
The present invention relates to a composite hollow fiber membrane having an excellent peeling strength, filtration reliability, and water permeability.
Recently, polymer separation membranes are being utilized in more various fields as well as existing application fields with the improvement of their techniques. Particularly, with the importance of environment, demands for them are being increased in the fields of water treatment. In all application fields of separation membranes, a mechanical strength, such as peeling strength, always stands out as an important factor as well as selectiveness and water permeability. Particularly, in water treatment fields, an excellent mechanical strength is necessarily required, simultaneously with a high permeability, from the viewpoint of the reliability of a separation membrane system.
2. Background Art
A hollow fiber-shaped membrane has a high permeability per installation area and is suitable for water treatment, whereas the mechanical strength thereof has been a problem to be solved due to the characteristics of a porous membrane structure. Thus, a hollow fiber membrane reinforced with a fabric or tubular braid having an excellent mechanical strength as a support of the separation membrane. Such a general idea of a composite membrane is a well known fact.
Techniques thereof are disclosed in U.S. Pat. No. 4,061,821, U.S. Pat. No. 3,644,139, U.S. Pat. No. 5,472,607,U.S. Pat. No. 6,354,444 and the like. Among them, a general idea of a composite hollow fiber membrane using a tubular braid was disclosed for the first time in U.S. Pat. No. 4,061,821 to Hayano et al. In this technique, however, the tubular braid is not used as a support for coating, but it is completely embedded in the membrane in order to compensate for a reduction of water permeability due to the shrinkage occurred when an acryl hollow fiber type membrane is solely used at a temperature higher than 80° C. Such a composite membrane has a larger thickness than the thin film coated on a support, and the embedded braid increases the resistance of fluid flow for thereby significantly reducing the water permeability.
Unlike the prior art, in U.S. Pat. No. 5,472,607, a reinforcing material is not embedded in the membrane, but is coated on its surface with a thin film by coating method of the existing flat composite membrane. In manufacturing a composite hollow fiber membrane having a thin film layer coated on the surface of a reinforcing material or supporting material of a tubular braid, thermodynamic stability differs according to the composition of a dope to be used for coating. This determines the structure of the coated thin film layer.
That is to say, in case of a thermodynamically stable dope, it has a finger-like structure. On the contrary, a dope with a low thermodynamic stability has a sponge structure with no defect region. For instance, in case of a dope used a solvent having a strong solvent power such as N-methyl-2-pyrrolidone (NMP) among organic solvents, it can easily form a finger-type structure because it has a high thermodynamic stability.
Additionally, the water permeability and mechanical strength of the overall composite hollow fiber membrane depends upon the structure and properties of the thin film layer. This is because the thin film layer has small pores and a low mechanical strength than a tubular braid reinforcing material having relatively much larger pores and a higher strength. In other words, the filtrate having passed through the thin film layer passes through a braid supporting layer with relatively large pores without a large resistance. While, since the thin film layer has a large flow resistance, the water permeability of the overall membrane is determined according to a microporous structure and porosity.
In view of strength, the tensile strength, pressure resistance and the like are complemented by the braid reinforcing material having a far superior mechanical strength. However, if the strength of the thin film is reduced, the thin film is separated or damaged.
In U.S. Pat. No. 4,061,821 and U.S. Pat. No. 5,472,607, the significance of the coated thin film layer structure was overlooked in relative to the present invention. Particularly, the structure of the thin film layer in the two prior arts has a porous region larger than 5 μm in an inner layer of a skin, that is, the inner layer has some micro pores having a pore diameter larger than 5 μm.
FIG. 2 is an exploded sectional view of a composite hollow fiber membrane disclosed in U.S. Pat. No. 4,061,821; and FIG. 3 is an exploded sectional view of a composite hollow fiber membrane disclosed in U.S. Pat. No. 5,472,607. These membranes are in a finger-like structure as shown in FIGS. 2 and 3 and have a defect region D functioning as a defect in the thin film layer.
As seen from the well-known fact, they can acts as a defect in expressing the mechanical properties of the thin film. Particularly, when the skin of a dense layer is damaged, a material capable of being secondarily cut off by the inner layer is permeated. This reduces the filtration reliability of the membrane relatively.
The composite hollow fiber membrane is suitable, particularly for filtration modules in the fields of water treatment due to its superior mechanical strength. In such a filtration module, there is a possibility of damaging the surface of the membrane by the friction and physical impact generated between membranes due to aeration. Particularly, filtration by the inner layer is required so as to ensure high filtration reliability.
Meanwhile, U.S. Pat. No. 6,354,444 proposes a composite hollow fiber membrane coated with a polymer resinous thin film on a braid made composite hollow fiber membrane, the braid is made of monofilaments of 0.5 or higher denier without crimp, so the surface area of the braid contacted with the polymer resinous thin film is small, which leads to a low peeling strength between the braid and the polymer resin coated on its surface.